Where does the PV power go when a MPPT doesn't need it?

[AlaskanNoob]

I'm assuming PV puts out whatever power it can put out to the MPPT (and that it's not based on a demand set by the MPPT) and the MPPT then clips power it doesn't need (if the batteries are full and there is no inverter demand). If that assumption is correct, where does the extra power go that isn't used by the MPPT?

If my assumption is incorrect, how do MPPTs log PV available?

 

[cs1234]

I may be mistaken, but this is my understanding :

The MPPT pulls the power that it wants. If it doesn't pull what's available from the panels, it turns into HEAT in the panel.

As to logging unused available power, you got me on that one, I didn't know any of them did that.

 

[justgary]

The panel voltage will rise toward the open circuit voltage if the charge controller is not using it. Since the current goes to zero, the power output of the panel goes to zero as well (power equals voltage times current). The rest of the available power goes to the same place it would go if a panel wasn't sitting in that patch of the sunlight; it simply warms whatever is illuminated.

 

[MurphyGuy]

I'm assuming PV puts out whatever power it can put out to the MPPT (and that it's not based on a demand set by the MPPT) and the MPPT then clips power it doesn't need (if the batteries are full and there is no inverter demand). If that assumption is correct, where does the extra power go that isn't used by the MPPT?

If my assumption is incorrect, how do MPPTs log PV available?

The power doesn't go anywhere because it never gets produced.

If you put a volt meter on the isloated pos/neg terminals of a panel, you get maximum voltage.. because no current is flowing.

Likewise, if you put a current meter between pos and neg (a direct short), you will get maximum current flow, but almost no voltage.

The difference between those two extremes is the power point along the panel's energy production curve. The Maximum Power Point Tracking system will optimize a balance between voltage and current to get as much energy (watts) as it can from the panel, until such time as it no longer needs that energy, like when your batteries are full.. at that point, it will slow the current down by increasing voltage.

 

[AlaskanNoob]

I may be mistaken, but this is my understanding :

The MPPT pulls the power that it wants. If it doesn't pull what's available from the panels, it turns into HEAT in the panel.

As to logging unused available power, you got me on that one, I didn't know any of them did that.

I just saw some charts that some Victron MPPTs produce, power over last 30 days and such and assumed that's what it was showing. Could be those charts were actually showing only power that was used and available from the PV instead of power the PV could provide.

 

[AlaskanNoob]

Thanks for the posts about how the power is produced from the panels. Far more complicated than I was thinking.

 

[AlaskanNoob]

Just watched this video and touched on the power curve that was mentioned in this thread. It seems the MPPT algorithm uses a "trial and error" method of varying current (so I guess it is setting a demand) and then measuring the power that is produced, to try to find the max power point. So if that's the case, I guess the MPPT doesn't really know the max the PV could produce at any given moment and at times when it wants less power for topping off the batteries and such, it's just going to lower the current demand to get it (and when it's doing that, perhaps a cloud has appeared and what the MPPT wants is all the PV could produce at that point, or maybe there is no cloud and the MPPT wouldn't know, it just gets what it needs).

So I guess for trying to log what kind of power would be available you'd need something other than the MPPT to measure that.

 

[The Good Package]

I just saw some charts that some Victron MPPTs produce, power over last 30 days and such and assumed that's what it was showing. Could be those charts were actually showing only power that was used and available from the PV instead of power the PV could provide.

I use Victron SmartSolar MPPT units and they track maximum USED watts (P max) and maximum detected voltage (V max). They get really warm and smell like burning robots when you run them at 100% for prolonged periods of time, so I aim for 20% over max usable watts. For example, 1200w of solar for a unit that has a max of about 1000w. This keeps it at about 75-85% max charging amps on a typical sunny day and no robot smell.

 

[Mattb4]

I'm assuming PV puts out whatever power it can put out to the MPPT (and that it's not based on a demand set by the MPPT) and the MPPT then clips power it doesn't need (if the batteries are full and there is no inverter demand). If that assumption is correct, where does the extra power go that isn't used by the MPPT?

If my assumption is incorrect, how do MPPTs log PV available?

The word 'clip' or clipping while frequently used by people is a poor choice (and really should be abandoned) that leads people to think power is still being produced but not utilized. Think of it like plugging something into your 120vAC 15 amp wall outlet. Your potential available power is 1800w (above which circuit protection cuts in.) but you only see demand wattage from the device you are using. Lets say a 100w light bulb. Is the circuit clipping that unused 1700w? No it is simply not using more than the load needs.Loads drive supply needs

PV solar panels produce power while in sunlight when a load is applied. If there is no load there is no current flow. At this point they are no different than any other object sitting in the sun with the exception that potential (voltage) is occurring.

No MPPT SCC can log potential power. Potential power depends on amount of sunlight, ambient temperature, and total area of cells (plus cell efficiency). None of which the MPPT SCC knows anything about.

To answer the Thread title. The PV power does not go anywhere because it does not exist.

 

[cs1234]

So I guess for trying to log what kind of power would be available you'd need something other than the MPPT to measure that.

You need a bigger load, like more batteries or things plugged into the wall using power. You also need to make sure the MPPT has more wattage capability than the panels, so it's not "clipping" the panels. Then, if your load matches or exceeds the amount the panels can produce, you will get your maximum available solar power number from those panels. The maximum those panels and MPPT can produce anyway.